1. Field of the Invention
This invention relates to vectoring thrust and changing the throat area of a ducted propulsion system. More particularly, the invention comprises two arms rotatably mounted along an interior wall of a choked flow nozzle which, when rotated, vector engine thrust and vary the throat area of the propulsion system.
2. Description of the Prior Art
Unless its trajectory is ballistic, a flight vehicle must have means for changing its direction of travel and, in particular, its rotation about the yaw axis. When a flight vehicle is powered by a jet engine, the performance characteristics of the jet engine are of prime importance. These two fundamental tenets play significant roles in the design and performance of every flight vehicle powered by a jet engine.
The performance of a jet engine is directly affected by the throat area of the exhaust nozzle. Performance is maximized when the area of the choke plane of the engine's exhaust nozzle approximates the theoretical optimal value that can be calculated using equations well known in the art of fluid mechanics. Simply stated, the optimal choke plane area will vary as a function of the altitude, Mach number and power setting at which the jet engine is operating. Since these parameters vary throughout a typical flight, it is not possible for a jet engine to be operating at maximum efficiency throughout the entire flight envelope of a flight vehicle unless the throat area can be varied.
One means to enhance jet engine efficiency is to change the throat area by means of a rotatable conical plug located in the nozzle throat. An example of such a device is shown in U.S. Pat. No. 3,907,222 issued to Tommy D. McComas. This approach reduces the thrust otherwise generated because the plug obstructs the fluid medium flowing through the nozzle.
Rotation about the yaw, pitch or roll axis is commonly controlled by having a control surface projecting from the fuselage and exposed to the ambient wind. The control surface is deflected to create the desired moment about the vehicle's center of gravity. This invention is directed to controlling yaw. However, as will subsequently become clear, the invention could alternatively be used to control pitch if its axis of rotation were rotated by 90.degree. from the orientation shown in the described embodiment.
Yaw has typically been controlled by a vertical stabilizer having a hinged rudder, where the rudder comprises the aft section of the stabilizer. The stabilizer and rudder project radially outward from the external skin of the flight vehicle. Use of this apparatus causes an increase in the skin friction drag and pressure drag for the vehicle compared to what the magnitude of these drag components would be if the vehicle had no such projecting structure, with the drag increasing in proportion to the amount of deflection.
The surface area of the rudder must be adequate to provide sufficient force to control yaw at the minimum flight velocity of the vehicle. However, this area is more than enough to control the vehicle at higher velocities, and thus results in excessive drag at velocity greater than the minimum flight velocity. The rudder must also have sufficient area to provide the vehicle with the desired degree of maneuverability. Again, designing the rudder to have the capability of providing the desired degree of maneuverability penalizes the vehicle with drag when the capability to maneuver is not being fully utilized.
The advent of the jet engine has been accompanied by numerous efforts to implement directional control by vectoring the thrust loads produced by such an engine. The common approach is to place a hinged rudder or rotatable vane in the expansion shroud downstream of the throat. This provides a smoother aerodynamic shape for the external skin. However, the rudder or rotatable vane is located in the supersonic exhaust flow downstream of the sonic choke plane. This placement creates shock waves which reduce the thrust and the resultant control force which would otherwise be generated by a given deflection of the rudder or vane.
The drawbacks caused by the shock waves created by a rotatable vane in the nozzle can be avoided by placing the vane in the throat, as shown in U.S. Pat. No. 3,743,184 issued to H. Vincent Mancus. However, positioning a rotatable vane in the nozzle throat causes variation of the effective throat area and results in fluctuations of the static pressure whenever the vane is rotated to vector thrust. More particularly, suddenly reducing the throat area in the nozzle of a jet engine will cause a rapid increase in the static pressure upstream of the throat. If such a device is used with an air breathing turbine propulsion system, the sudden increase in static pressure downstream of the turbine machinery can stall airflow across the turbine blades and cause an instantaneous degradation of engine performance.
Conversely, if the effective throat area is rapidly increased by the thrust vectoring mechanism, the static pressure will quickly drop and reduce the turbine efficiency, with a resultant degradation of engine performance. In summary, the use of a rotatable vane in the throat of the exhaust nozzle of an air breathing jet engine to vector its thrust will cause the performance of the engine to wander. Depending on the rates at which the vane is rotated, these performance variations may be uncontrollable.
Thrust vectoring is also achieved by complex mechanical devices which swivel the exhaust shroud of the nozzle so that the flow is deflected to exhaust in the desired direction. Such apparatus cause an increase in drag. They also are very complex and relatively heavy.
Several devices perform the dual functions of thrust vectoring and varying the area of the nozzle throat by deflecting the fluid flow with a vane or flap located in the nozzle, and concurrently changing the throat area by moving an obstruction located in the throat to change its projected planar area. Examples of such devices are shown in U.S. Pat. No. 3,970,253 issued to William M. Burkes and William H. Miller, and U.S. Pat. No. 3,774,868 issued to Gerald F. Goetz. These devices are combinations of prior art apparatus which are used independently to achieve the two functions, and thus suffer the aforementioned thrust and weight penalties attendant to the use of such apparatus.
The applicants have also invented the thrust vectoring device disclosed in U.S. Pat. No. 5,092,524 entitled Nozzle Throat Disc for Thrust Vectoring. The apparatus described therein is a symmetrical disc located in the throat of a nozzle. The disc is rotatable about an axis which passes through its center and lies normal to the upstream fluid flow. A rounded bump runs radially along a disc diameter and projects from both major surfaces. When the disc is rotated so that the diametrical bump is not perpendicular to the upstream fluid flow, the fluid flow turns upon passing over the bump to flow in a direction approximately normal to the bump. This device overcomes the aforementioned problems and drawbacks attendant to prior art devices for vectoring thrust, but does not solve the problem of changing the throat area in order to maximize engine performance at various altitudes and fluid velocities.